The invention relates to a control device for a laser diode. The control device includes a bias direct current source supplying a bias direct current to the laser diode. The control device includes a modulator for modulating a signal current for the laser diode in accordance with a data signal received by the modulator. The control device also includes a temperature compensation device that can be connected to the modulator such that the signal current can be regulated and/or controlled as a function of temperature changes.
Laser diodes are used for the fast transmission of digital, optical signals, in particular, because they have a narrow spectral width and the capacity for fast modulation. In the case of such laser diodes, deviations from a predefined characteristic curve occur during operation, depending for example, on the age of the laser diode or on the temperature. Depending on how the laser diode is used, various control mechanisms are used in order to compensate for the deviations from the characteristic curve.
These control mechanisms include using a temperature compensation device. Regulation of this type is necessary since the current threshold value Is of the characteristic curve of a laser diode, at which the characteristic curve changes from a region of the bias direct current (no light emission) into a region of the signal current (light emission region), is temperature dependent. Using the temperature compensation device, the bias direct current of the laser diode can be regulated in such a way that the laser diode always operates in the region of the signal current. The disadvantage of this type of control is that no regulation of the modulated signal current is carried out, so that, for example, changes in the characteristic curve, in particular with regard to the slope as a consequence of the aging of the laser diode, or on account of temperature changes, cannot be compensated for. The influence of such characteristic curve changes can be reduced, preferably ruled out, by using a pilot tone control device.
A pilot tone control device is known, for example, from the publication by D. W. Smith and T. G. Hodgkinson entitled, xe2x80x9cLaser level control for optical fiber systems with high bit ratexe2x80x9d, symposium, Houston, April 1980. A distinction is drawn between pilot tone control to the high level of the signal current from the laser diode and pilot tone control to the high and the low level of the signal current. In principle, in this case a pilot tone frequency is modulated onto the signal current (pilot tone control on the high level) or onto the signal current and the direct current (pilot tone control on the high and the low level). Using a monitor diode, part of the light emitted is registered. The registered part of the light is evaluated in order to carry out a readjustment in the event of deviations with regard to the mean output power of the laser diode and the pilot tone amplitude. The disadvantage with pilot tone control to the high and the low level is that at high data rates, because of the modulation of the signal current in the region of the threshold current Is, at which the characteristic curve of the laser diode bends, jitter is promoted. This can be avoided by using pilot tone control at the high level of the signal current.
It is accordingly an object of the invention to provide a control device for a laser diode which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
In particular, it is an object of the invention to provide a control device for a laser diode with which the possibilities of using the various control mechanisms are improved.
With the foregoing and other objects in view there is provided, in accordance with the invention, a control device for at least one laser diode, including: a bias direct current source connected to the laser diode and providing a bias direct current to the laser diode; a modulator for modulating a signal current for the laser diode in accordance with a data signal received by the modulator; a temperature compensation device switchably connected to the modulator such that the signal current can be operated on, as a function of temperature changes, in a manner selected from the group consisting of being regulated and being controlled; a pilot tone control device for controlling the signal current by modulating a pilot tone frequency onto the signal current; and a first switching device for switching between a first state and a second state. In the first state, the pilot tone control device is connected to the modulator and the bias direct current source such that the signal current can be regulated using the pilot tone control device. In the second state, the modulator is connected to the temperature compensation device such that the temperature compensation device operates on the signal current in a manner selected from the group consisting of regulating the signal current and controlling the signal current.
In accordance with an added feature of the invention, the pilot tone control device includes a second switching device for changing the pilot tone control device between a state for controlling at a high level of the signal current, and another state for controlling at a low level of the signal current and at the high level of the signal current.
In accordance with an additional feature of the invention, the second switching device includes programmable switches.
In accordance with another feature of the invention, the second switching device is controlled using a switching signal having a low level and a high level.
In accordance with a further feature of the invention, the pilot tone control device includes: a monitor diode for receiving a monitor signal; a first integrator connected to the monitor diode and to the bias direct current source; an oscillator for generating the pilot tone frequency; a connection between the monitor diode and the first integrator; a frequency mixing device coupled to the connection between the monitor diode and the first integrator, the frequency mixing device coupled to the oscillator; a conditioning device receiving the pilot tone frequency from the oscillator and conditioning the pilot tone frequency, the conditioning device having a first output switchably connected to the modulator, the conditioning device having a second output switchably connected to the bias direct current source; and a second integrator connected to the frequency mixing device, the second integrator having an output switchably connected to the modulator.
In accordance with a further added feature of the invention, the second integrator has an input; the second switching device has a switching state connecting the input of the second integrator to the reference voltage device to set the pilot tone control device into the state for controlling at the high level of the signal current; and the second switching device has another switching state connecting the input of the second integrator to a ground potential to set the pilot tone control device into the other state for controlling at the low level of the signal current and at the high level of the signal current.
In accordance with another added feature of the invention, the first switching device includes a first switch coupled between the output of the second integrator and the modulator; the first switching device includes a second switch coupled between the first output of the conditioning device and the modulator; and the first switching device includes a third switch coupled between the second output of the conditioning device and the bias direct current source.
In accordance with yet an added feature of the invention, there is provided, a reference voltage device, and a second switching device. The second integrator has an input. The pilot tone control device has two states. The second switching device has a first switching state and a second switching state. In the first switching state, the second switching device connects the input of the second integrator to the reference voltage device to set the pilot tone control device into one of the two states of the pilot tone control device. In the second switching state, the second switching device connects the input of the second integrator to a ground potential to set the pilot tone control device into another one of the two states of the pilot tone control device.
In accordance with yet an additional feature of the invention, the control device controls a plurality of laser diodes.
In accordance with yet another feature of the invention, the first switching device includes programmable switches.
In accordance with an added feature of the invention, the first switching device is controlled using a switching signal having a low level and a high level.
The object of the invention is achieved by providing a pilot tone control device for controlling the signal current by modulating a pilot tone frequency onto the signal current, and also by providing a switching device for switching the control device between one state and another state. In one state, the pilot tone control device is connected to the modulator and the bias direct current source such that the signal current can be regulated using the pilot tone control device. In the other state, the modulator is connected to the temperature compensation device such that the signal current can be regulated and/or controlled using the temperature compensation device.
The substantial advantage achieved with the invention as compared with the prior art consists in the fact that by using a single control device, the benefit of two of the above-described different control mechanisms for laser diodes can be used. Depending on the particular application, it is therefore possible for the control mechanism beneficial to the application to be selected and used.
In an expedient development of the invention, the pilot tone control device includes a second or further switching device, so that the pilot tone control device can be switched between a state in which it is possible to control at a high level of the signal current and another state in which it is possible to control at both the low level and the high level of the signal current. In this way, when using a single control device, it is possible to select between three possible control mechanisms.
One advantageous refinement of the invention provides for the pilot tone control device to include the following components: a monitor diode for receiving a monitor signal; a first integrator that is connected to the monitor diode and the bias direct current source; a frequency mixing device that is coupled to a connection between the monitor diode and the first integrator and to an oscillator device for producing the pilot tone frequency; and a conditioning device for conditioning the pilot tone frequency received from the oscillator device. It is possible for one output of the conditioning device to be connected to the modulator, and for another output of the conditioning device to be connected to the bias direct current source. A second integrator is connected to the frequency mixing device and has an output that can be connected to the modulator. This makes it possible to construct an embodiment of the control device by using simple circuit devices.
Expediently, one input of the second integrator can be connected to the further switching device. The input can be coupled to a reference voltage device or to a ground potential using the further switching device, as a function of a switching state of the further switching device, in order to be able to set the pilot tone control device into one state or the other state. By this arrangement, and by using simple components, a possible way of switching the pilot tone control device between two states is provided.
In a development of the invention, the switching device and/or the further switching device are constructed as programmable switches, which provides a possible route for externally controlling the switching device and the further switching device.
The switching device and/or the further switching device can advantageously be controlled using a high or a low level of a switching signal. This makes it possible to control the switching device and the further switching device using simple circuit components.
A development of the invention provides for the switching device to include a first switch coupled between the output of the second integrator and the modulator. The switching device also includes a second switch coupled between one output of the conditioning device and the modulator, and a third switch coupled between another output of the conditioning device and the bias direct current source. By this arrangement, the components of the pilot tone control device can substantially be separated completely from the control device, so that in the case of using temperature compensation, disruptive influences from the components of the pilot tone control device are reduced.
In an expedient refinement of the invention, the laser diode is replaced by an arrangement of a plurality of laser diodes, as a result of which, the control device can be used, for example, for laser diode arrangements for transmitting a plurality of parallel, optical signals.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a control device for laser diodes, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.